Semiconductor integrated circuit

ABSTRACT

A semiconductor integrated circuit includes a cascode circuit having a transistor, a detector circuit and a bias generator circuit. A bias is applied to a substrate of the transistor. The detector circuit generates a signal related to a threshold voltage of the transistor. The bias generator circuit generates the bias based on the signal generated by the detector circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-194104, filed on Aug. 25, 2009, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments discussed herein relate to semiconductor integrated circuits.

BACKGROUND

Recently, the speed and channel capacity of communication networks have been steadily increasing. The scale of circuits performing amplification and waveform-shaping of high-speed electrical signals exchanged over such communication network tends to increase. On the other hand, there is a need to decrease power consumption of the circuits. Thus, strict conditions are imposed on design of such circuits. Meanwhile, to perform amplification and waveform-shaping of high-speed electrical signals, such circuits often have, for example, a multi-stage configuration in which a plurality of amplifier circuits are connected in series (see, for example, Japanese Unexamined Patent Application Publication Nos. 2008-236515 and 2006-279599).

However, in the foregoing related art, when threshold voltages of transistors in circuits fluctuate because of process variation of the transistors, the fluctuation unfortunately decreases a margin of circuit design. The fluctuation of the threshold voltages of the transistors makes direct-current (DC) potential design difficult particularly in semiconductor integrated circuits achieving both the amplification and waveform-shaping of high-speed electronic signals and the decrease in power consumption.

SUMMARY

According to an aspect of the invention, a semiconductor integrated circuit includes a cascode circuit having a transistor, a detector circuit and a bias generator circuit. A bias is applied to a substrate of the transistor. The detector circuit generates a signal related to a threshold voltage of the transistor. The bias generator circuit generates the bias based on the signal generated by the detector circuit.

Advantages of the invention will be realized and attained via the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating a semiconductor integrated circuit according to a first example embodiment of the invention;

FIG. 2 is a graph illustrating an example of fluctuations of a threshold voltage of a transistor due to process variation;

FIG. 3 is a diagram illustrating an example of a potential value at each point of a semiconductor integrated circuit affected by process variation;

FIG. 4 is a graph illustrating a change in a threshold voltage against a bias applied to a substrate of a transistor;

FIG. 5 is a graph illustrating a change in a potential at a point B illustrated in FIG. 1 against a bias;

FIG. 6 is a diagram illustrating an example of a parallel/serial converter including the semiconductor integrated circuit illustrated in FIG. 1;

FIG. 7 is a circuit diagram illustrating a semiconductor integrated circuit according to a second example embodiment of the invention;

FIG. 8 is a circuit diagram illustrating a modification of the semiconductor integrated circuit illustrated in FIG. 7;

FIG. 9 is a circuit diagram illustrating a semiconductor integrated circuit according to a third example embodiment of the invention;

FIG. 10 is a circuit diagram illustrating a modification of the semiconductor integrated circuit illustrated in FIG. 9;

FIG. 11 is a graph illustrating a characteristic of current flowing through a substrate of a transistor against a bias;

FIG. 12 is a circuit diagram illustrating a first example of other semiconductor integrated circuits; and

FIG. 13 is a circuit diagram illustrating a second example of other semiconductor integrated circuits.

DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will be described below with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a circuit diagram illustrating a semiconductor integrated circuit according to a first embodiment. As illustrated in FIG. 1, a semiconductor integrated circuit 100 according to the first embodiment includes a power supply 101, a path 102 a, a path 102 b, a first amplifier circuit 110, a second amplifier circuit 120, a detector circuit 131, and a bias generator circuit 132. The power supply 101 supplies voltage to the first amplifier circuit 110 and the second amplifier circuit 120. A signal amplified by the first amplifier circuit 110 is supplied to the second amplifier circuit 120 through the paths 102 a and 102 b.

The first amplifier circuit 110 includes a resistor 111, resistors 112 a and 112 b, transistors 113 a and 113 b, a current source 114, and input terminals 115 a and 115 b. One end of the resistor 111 is connected to the power supply 101, whereas the other end thereof is connected to the resistors 112 a and 112 b. One end of the resistor 112 a is connected to the resistor 111, whereas the other end thereof is connected to a drain of the transistor 113 a. One end of the resistor 112 b is connected to the resistor 111, whereas the other end thereof is connected to a drain of the transistor 113 b.

The transistors 113 a and 113 b are included in a differential pair. The drain of the transistor 113 a is connected to the resistor 112 a. A source of the transistor 113 a is connected to the current source 114, whereas a gate of the transistor 113 a is connected to the input terminal 115 a. The drain of the transistor 113 b is connected to the resistor 112 b. A source of the transistor 113 b is connected to the current source 114, whereas a gate of the transistor 113 b is connected to the input terminal 115 b.

One end of the current source 114 is connected to the sources of the transistors 113 a and 113 b, whereas the other end thereof is connected to ground. The path 102 a to the second amplifier circuit 120 is connected between the resistor 112 a and the transistor 113 a. The path 102 b to the second amplifier circuit 120 is connected between the resistor 112 b and the transistor 113 b.

Signals input from the input terminals 115 a and 115 b are applied to the gates of the transistors 113 a and 113 b, respectively. In accordance with the input signals, current flowing from the resistors 112 a and 112 b to the current source 114 is generated. Accordingly, amplified signals are output to the second amplifier circuit 120 through the paths 102 a and 102 b.

The second amplifier circuit 120 includes resistors 121 a and 121 b, transistors 122 a and 122 b, power supplies 123 a and 123 b, transistors 124 a and 124 b, a current source 125, and output terminals 126 a and 126 b. One end of the resistor 121 a is connected to the power supply 101, whereas the other end thereof is connected a drain of the transistor 122 a. One end of the resistor 121 b is connected to the power supply 101, whereas the other end thereof is connected to a drain of the transistor 122 b.

The transistors 122 a and 122 b are included in a differential pair. The drain of the transistor 122 a is connected to the resistor 121 a. A source of the transistor 122 a is connected to a drain of the transistor 124 a, whereas a gate of the transistor 122 a is connected to the power supply 123 a. The drain of the transistor 122 b is connected to the resistor 121 b. A source of the transistor 122 b is connected to a drain of the transistor 124 b, whereas a gate of the transistor 122 b is connected to the power supply 123 b.

The transistors 124 a and 124 b are included in a differential pair. The drain of the transistor 124 a is connected to the source of the transistor 122 a. A source of the transistor 124 a is connected to the current source 125, whereas a gate of the transistor 124 a is connected to the path 102 a. The drain of the transistor 124 b is connected to the source of the transistor 122 b. A source of the transistor 124 b is connected to the current source 125, whereas a gate of the transistor 124 b is connected to the path 102 b.

Each of the transistors 124 a and 124 b may be, for example, a metal oxide semiconductor (MOS) transistor. Additionally, each of the transistors 113 a, 113 b, 122 a, and 122 b may be, for example, a MOS transistor. A bias (substrate bias) output from the bias generator circuit 132 is applied to substrates of the transistors 124 a and 124 b.

The output terminal 126 a is connected between the resistor 121 a and the drain of the transistor 122 a. The output terminal 126 b is connected between the resistor 121 b and the drain of the transistor 122 b. Signals output from the first amplifier circuit 110 are applied to the gates of the transistors 124 a and 124 b through the paths 102 a and 102 b, respectively. In accordance with the signals from the first amplifier circuit 110, current flowing from the resistors 121 a and 121 b to the current source 125 is generated. Accordingly, amplified signals are output from the output terminals 126 a and 126 b.

The transistor 122 a and 122 b and the power supplies 123 a and 123 b are inserted so that the transistors 124 a and 124 b form cascode configurations, respectively, whereby an operation speed of the second amplifier circuit 120 is increased. In the cascade configurations, the transistor 122 a is stacked with the transistor 124 a and the transistor 122 b is stacked with the transistor 124 b. In this case, potential voltages (hereafter, “potentials”) at points (called out by the label “C” in FIG. 1) between the transistors 122 a and 124 a and between the transistors 122 b and 124 b are lower than those at the output terminals 126 a and 126 b, respectively.

To acquire a drain voltage sufficient for driving the transistors 124 a and 124 b, a point (point B) between the current source 125 and the transistors 124 a and 124 b is designed to have a low potential. More specifically, the potential at the point B is lower than potentials (gate voltages of the transistors 124 a and 124 b) at points (points A) in the paths 102 a and 102 b by threshold voltages V_(th) of the transistors 124 a and 124 b, respectively. The threshold voltage V_(th) corresponds to a gate voltage for allowing current to flow between the source and the drain.

The resistor 111 is inserted on a downstream side of the power supply 101 in the first amplifier circuit 110 so that the point B has a low potential. For example, when the power supply 101 has a supply voltage of 1.2 V, a DC potential V_(a) at the points A can be represented by Equation (1) using a resistance value R_(c) of the resistor 111, a resistance value R_(o) of the resistors 112 a and 112 b, and a current value I_(ref) of the current source 114.

$\begin{matrix} {V_{a} = {1.2 - \left( {I_{ref} \times {Rc}} \right) - \left( {\frac{I_{ref}}{2} - R_{o}} \right)}} & (1) \end{matrix}$

When the resistance value R_(c) of the resistor 111, the resistance value Ro of the resistors 112 a and 112 b, and the current value I_(ref) of the current source 114 are, for example, equal to 8 μg, 30 μg, and 20 mA, respectively, in Equation (1), the DC potential Va at the points A is equal to 0.74 V. When the threshold voltage V_(th) of each of the transistors 124 a and 124 b is equal to 0.64 V, the potential at the point B is equal to 0.1 V. Accordingly, little margin is left in low DC potential design at the point B.

In such a configuration, if the threshold voltage V_(th) of each of the transistors 124 a and 124 b fluctuates because of process variation, the fluctuation decreases the margin in the DC potential design at the point B. For example, if the threshold voltages V_(th) of the transistors 124 a and 124 b increase because of process variation, the potential at the point B approaches 0 V, and a voltage for driving the current source 125 becomes insufficient. Accordingly, the DC potential design of the second amplifier circuit 120 fails.

However, in the semiconductor integrated circuit 100, the detector circuit 131 and the bias generator circuit 132 compensate the fluctuation of the threshold voltages V_(th) of the transistors 124 a and 124 b due to process variation. More specifically, the detector circuit 131 detects the threshold voltages of the transistors 124 a and 124 b and then outputs a signal indicative thereof as a detection result to the bias generator circuit 132.

The bias generator circuit 132 generates a bias on the basis of the detection result supplied from the detector circuit 131. The bias generator circuit 132 outputs the generated bias. The bias output from the bias generator circuit 132 is applied to a substrate of each of the transistors 124 a and 124 b. A substrate bias effect occurs in the transistors 124 a and 124 b in response to application of the bias to the substrates of the transistors 124 a and 124 b, respectively.

More specifically, changes in widths of depletion layers of the transistors 124 a and 124 b change substantial threshold voltages of the transistors 124 a and 124 b, respectively. In this manner, the fluctuation of the threshold voltages V_(th) of the transistors 124 a and 124 b due to process variation can be compensated. Thus, for example, the margin (e.g., a measure of the difference between a worst-case performance of a design and a performance goal for the design) can be increased in DC potential design at the point B.

FIG. 2 is a graph illustrating an example of fluctuations of a threshold voltage of a transistor due to process variation. In semiconductor manufacturing, a process corner is an example of a design-of-experiments (DoE) technique that refers to a variation of fabrication parameters used in applying an integrated circuit design to a semiconductor wafer. Process corners represent extreme instances of these parameter variations within which a circuit that has been etched onto the wafer should function correctly. A circuit running on devices fabricated at these process corners may run slower or faster than specified and at lower or higher temperatures and voltages, but if the circuit does not function at all at any of these process extremes then the design is considered to have inadequate design margin. Referring to FIG. 2, the horizontal axis represents a voltage (gate voltage) applied to the gate of each of the transistors 124 a and 124 b, whereas the vertical axis represents current (drain current) flowing through the drain of each of the transistors 124 a and 124 b.

One naming convention for process corners uses two-letter designators, where the first letter refers to the NMOS corner, and the second letter refers to the PMOS corner. In FIG. 2, a line 211 represents a characteristic of the drain current against the gate voltage of the transistors 124 a and 124 b resulting from a typical-typical (TT) process corner, namely a corner without substantial process variation, i.e., a corner that does not exhibit substantial process variation. A line 212 represents a characteristic of the drain current against the gate voltage of the transistors 124 a and 124 b resulting from a slow-slow (SS) process corner. A line 213 represents a characteristic of the drain current against the gate voltage of the transistors 124 a and 124 b resulting from a fast-fast (FF) process corner.

A value Vth1 represents the threshold voltage V_(th) (reference threshold voltage) of each of the transistors 124 a and 124 b resulting from the process TT. A value Vth2 represents the threshold voltage V_(th) of each of the transistors 124 a and 124 b resulting from the process SS. A value Vth3 represents the threshold voltage V_(th) of each of the transistors 124 a and 124 b resulting from the process FF.

As illustrated by the lines 211-213, the characteristic of the drain current against the gate voltage fluctuates because of the process variation (the processes TT, SS, and FF) of the transistors 124 a and 124 b. With this fluctuation of the characteristic, the threshold voltage V_(th) of each of the transistors 124 a and 124 b also fluctuates as shown by the values Vth1-Vth3.

FIG. 3 is a diagram illustrating an example of a potential value at each point of a semiconductor integrated circuit affected by process variation. In a table 300 illustrated in FIG. 3, a column “V_(th)” shows the threshold voltage V_(th) of each of the transistors 124 a and 124 b illustrated in FIG. 1. A column “A” shows potential values at the points A (see FIG. 1) of the semiconductor integrated circuit 100. A column “B” shows potential values at the point B (see FIG. 1) of the semiconductor integrated circuit 100. For example, when the semiconductor integrated circuit 100 is fabricated in the process TT, the threshold voltage V_(th) is equal to 0.64 V (reference threshold voltage).

When the semiconductor integrated circuit 100 is fabricated in the process SS, the threshold voltage V_(th) is, for example, equal to 0.74V, which is higher than the reference threshold voltage. The potential value at the point B of the semiconductor integrated circuit 100 is, for example, equal to 0.00 V, and, thus, little margin is left in the low DC potential design at the point B. In this case, compensation of the threshold voltage V_(th) increases the margin in the DC potential design at the point B.

When the semiconductor integrated circuit 100 is fabricated in the process FF, the threshold voltage V_(th) is, for example, equal to 0.50 V, which is lower than the reference threshold voltage. The potential value at the point B of the semiconductor integrated circuit 100 is, for example, equal to 0.24 V. When little margin is left in high DC potential design at the point B, compensation of the threshold voltage V_(th) can increase the margin in the DC potential design at the point B.

FIG. 4 is a graph illustrating a change in a threshold voltage against a bias applied to a substrate of a transistor. Referring to FIG. 4, the horizontal axis represents a bias V_(bs) applied to a substrate of each of the transistors 124 a and 124 b, whereas the vertical axis represents the threshold voltage V_(th) of each of the transistors 124 a and 124 b. A curve 400 represents a characteristic of the threshold voltage V_(th) against the bias Vbs.

As shown by the curve 400, the threshold voltages V_(th) of the transistors 124 a and 124 b decrease as the bias V_(bs) applied to the substrates of the transistors 124 a and 124 b increases, respectively. The threshold voltage V_(th) can be represented by, for example, Equation (2).

$\begin{matrix} {V_{th} = {V_{th\_ ref} + {\frac{\sqrt{2 \times q \times ɛ_{s} \times N_{a}}}{2}\left( {\sqrt{{2\varphi_{b}} + V_{bs}} - \sqrt{2\varphi_{b}}} \right)}}} & (2) \end{matrix}$

In Equation (2), a term V_(th) _(—) _(ref) represents the threshold voltage V_(th) obtained when no bias is applied to the substrates of the transistors 124 a and 124 b. Additionally, “q”, “

_(s)”, “N_(a)”, “Co”, and “2

b” represent an elementary charge, a dielectric constant of a semiconductor, an impurity concentration, an oxide film capacity per a unit area, and a surface potential for inversion, respectively. Here, “q” and “

_(s)” are constants, whereas “N_(a)”, “Co”, and “2

b” are determined in a fabrication process of the transistors 124 a and 124 b.

FIG. 5 is a graph illustrating a change in the potential at the point B illustrated in FIG. 1 against a bias. Referring to FIG. 5, the horizontal axis represents the bias V_(bs) [V] applied to the substrate of each of the transistors 124 a and 124 b, whereas the vertical axis represents the potential at the point B (point-B potential) [V] of the semiconductor integrated circuit 100 illustrated in FIG. 1. A curve 500 represents a characteristic of the point-B potential against the bias V_(bs) when the semiconductor integrated circuit 100 is fabricated in the process SS (where, e.g., the threshold voltage Vth2=0.74 V).

As illustrated by the curve 500, the point-B potential is nearly equal to 0 V when the bias V_(bs) is equal to 0 V. However, by setting the bias V_(bs) approximately equal to, for example, 0.6 V, the point-B potential can be set approximately equal to 0.07 V. In this manner, the margin can be increased in the DC potential design at the point B.

FIG. 6 is a diagram illustrating an example of a parallel/serial converter including the semiconductor integrated circuit illustrated in FIG. 1. Like reference characters are attached to configurations similar to those illustrated in FIG. 1 to omit the description. As illustrated in FIG. 6, a parallel/serial converter 600 includes input terminals 611-613, an amplifier circuit 614, a selector 620, the semiconductor integrated circuit 100, and an output terminal 630.

In the following discussion of FIG. 6, for the purposes of illustration, non-limiting examples of signals are mentioned. A 20-Gbps data signal (DATA1) is input to the input terminal 611, whereas a 20-Gbps data signal (DATA2) is input to the input terminal 612. For example, a 20-GHz clock signal (CLK) is input to the input terminal 613. The selector 620 alternately selects and outputs one of the data signals supplied from the input terminals 611 and 612 in synchronization with the clock signal supplied from the input terminal 613.

The data signal output from the selector 620 is input to the first amplifier circuit 110 of the semiconductor integrated circuit 100. The amplifier circuit 614 is connected between the input terminal 613 and the selector 620. The amplifier circuit 614 amplifies the clock signal supplied from the input terminal 613 and outputs the amplified clock signal to the selector 620.

The first amplifier circuit 110 amplifies the supplied data signal and then outputs the amplified data signal to the second amplifier circuit 120. The second amplifier circuit 120 amplifies the data signal supplied from the first amplifier circuit 110 and then outputs the amplified data signal from the output terminal 630. The data signal (DATA) output from the output terminal 630 is of 40 Gbps.

Illustration of the detector circuit 131 and the bias generator circuit 132 (see FIG. 1) of the semiconductor integrated circuit 100 is omitted in FIG. 6. Although the semiconductor integrated circuit 100 is a multi-stage circuit including the first amplifier circuit 110 and the second amplifier circuit 120, a sufficient margin is left in DC potential design by compensating the fluctuation of the threshold voltages of the transistors 124 a and 124 b due to process variation. Accordingly, power consumption of the semiconductor integrated circuit 100 can be decreased. Additionally, the sufficient margin in the DC potential design of the semiconductor integrated circuit 100 can improve flexibility of DC potential design of the selector 620 disposed on an upstream side of the semiconductor integrated circuit 100.

The semiconductor integrated circuit 100 according to the first embodiment detects the threshold voltages V_(th) of the transistors 124 a and 124 b and applies the bias generated on the basis of the detection result to substrates of the transistors 124 a and 124 b, respectively. In this manner, the semiconductor integrated circuit 100 compensates the fluctuation of the threshold voltage V_(th) due to process variation and, eventually, can increase a margin of circuit design.

In a circuit including the transistors 124 a and 124 b forming cascode configurations as illustrated in FIG. 1, the margin decreases in the low DC potential design. However, the semiconductor integrated circuit 100 can advantageously increase the margin of the circuit design by compensating the fluctuation of the threshold voltage V_(th) due to process variation. Additionally, when the transistors 124 a and 124 b are included in a downstream-side circuit (i.e., the second amplifier circuit 120) in a multi-stage circuit as illustrated in FIG. 1, the margin decreases in the low DC potential design. However, the semiconductor integrated circuit 100 can advantageously increase the margin of the circuit design by compensating the fluctuation of the threshold voltage V_(th) due to process variation.

Second Embodiment

FIG. 7 is a circuit diagram illustrating a semiconductor integrated circuit according to a second embodiment. In FIG. 7, like reference characters are attached to configurations similar to those illustrated in FIG. 1 to omit the description. As illustrated in FIG. 7, a semiconductor integrated circuit 100 according to the second embodiment includes a monitor transistor 711 functioning as the detector circuit 131 illustrated in FIG. 1 and a power supply 712.

The monitor transistor 711 monitors a threshold voltage V_(th) of each of transistors 124 a and 124 b. The monitor transistor 711 may be, for example, a MOS transistor having substantially the same current density as the transistors 124 a and 124 b. The monitor transistor 711 is formed on a chip that includes the transistors 124 a and 124 b. Additionally, a substrate of the monitor transistor 711 is connected to ground.

Accordingly, the monitor transistor 711 has a threshold voltage equal to the threshold voltages V_(th) of the transistors 124 a and 124 b obtained when no bias is applied to the substrates thereof. A source of the monitor transistor 711 is connected to ground. A gate of the monitor transistor 711 is connected to a drain of the monitor transistor 711. The drain of the monitor transistor 711 is connected to a bias generator circuit 132. Thus, the threshold voltage of the monitor transistor 711 is output to the bias generator circuit 132.

The power supply 712 outputs, to the bias generator circuit 132, a reference voltage V_(th) representing an idealized threshold voltage for each of the transistors 124 a and 124 b that would result from a process without process variation. The bias generator circuit 132 calculates and generates a bias on the basis of the threshold voltage output from the drain of the monitor transistor 711 and the reference voltage output from the power supply 712. For example, the bias generator circuit 132 calculates the bias using Equation (3) based on Equation (2).

$\begin{matrix} {V_{bs} = \frac{\left\{ {\left( {V_{thi} - {2\varphi_{b}}} \right)^{2} - \left( {V_{th\_ ref} - {2\varphi_{b}}} \right)^{2}} \right\}}{2 \times q \times ɛ_{s} \times \frac{N_{a}}{{Co}^{2}}}} & (3) \end{matrix}$

In Equation (3), the bias generator circuit 132 calculates a bias V_(bs) using the reference voltage V_(th) _(—) _(ref) output from the power supply 712 and the threshold voltage V_(thi) output from the drain of the monitor transistor 711. In this manner, the bias generator circuit 132 can generate the bias V_(bs) for compensating a fluctuation of the threshold voltage V_(th) of each of the transistors 124 a and 124 b due to process variation.

FIG. 8 is a circuit diagram illustrating a modification of the semiconductor integrated circuit illustrated in FIG. 7. In FIG. 8, like reference characters are attached to configurations similar to those illustrated in FIG. 7 to omit the description. As illustrated in FIG. 8, the semiconductor integrated circuit 100 may include a comparator circuit 811 in addition to the components illustrated in FIG. 7. The drain of the monitor transistor 711 is connected to the comparator circuit 811. The power supply 712 outputs the reference voltage to the comparator circuit 811.

The comparator circuit 811 compares the threshold voltage output from the drain of the monitor transistor 711 with the reference voltage output from the power supply 712. Upon determining that the threshold voltage is higher than the reference voltage, the comparator circuit 811 outputs the threshold voltage and the reference voltage to the bias generator circuit 132. Upon determining that the threshold voltage is not higher than the reference voltage, the comparator circuit 811 outputs neither the threshold voltage nor the reference voltage to the bias generator circuit 132.

Accordingly, the bias generator circuit 132 generates the bias if the comparator circuit 811 determines that the threshold voltage is higher than the reference voltage. However, the bias generator circuit 132 does not generate the bias if the comparator circuit 811 determines that the threshold voltage is not higher than the reference voltage. In this manner, the bias can be applied to substrates of the transistors 124 a and 124 b when the threshold voltages V_(th) of the transistors 124 a and 124 b are higher than the reference voltage, respectively.

The semiconductor integrated circuit 100 according to the second embodiment generates the bias calculated on the basis of the detected threshold voltage and the reference voltage to accurately compensate the fluctuation of the threshold voltage V_(th) due to process variation, thereby being able to increase a margin of circuit design. Additionally, the semiconductor integrated circuit 100 according to the second embodiment can accurately detect the threshold voltages of the transistors 124 a and 124 b by detecting the threshold voltage of the monitor transistor 711 included therein.

The monitor transistor 711 has substantially the same current density as the transistors 124 a and 124 b and is formed on a chip including the transistors 124 a and 124 b. With such a configuration, the fluctuation of the threshold voltage due to process variation of the transistors 124 a and 124 b is accurately reflected in the threshold voltage of the monitor transistor 711. Accordingly, the semiconductor integrated circuit 100 can accurately detect the threshold voltages of the transistors 124 a and 124 b by detecting the threshold voltage of the monitor transistor 711 and, eventually, can accurately compensate the fluctuation of the threshold voltage V_(th) due to process variation.

Third Embodiment

FIG. 9 is a circuit diagram illustrating a semiconductor integrated circuit according to a third embodiment. In FIG. 9, like reference characters are attached to configurations similar to those illustrated in FIG. 1 to omit the description. As illustrated in FIG. 9, a semiconductor integrated circuit 100 according to the third embodiment includes a power supply 911 and a feedback path 912 in addition to the components illustrated in FIG. 1. The power supply 911 outputs, to a bias generator circuit 132, a reference potential representing an idealized source voltage for each of transistors 124 a and 124 b that would result from a process without process variation.

The feedback path 912 for feeding a potential at a point B back to the bias generator circuit 132 functions as the detector circuit 131 illustrated in FIG. 1. The feedback path 912 allows the bias generator circuit 132 to acquire the source potential of each of the transistors 124 a and 124 b. The bias generator circuit 132 generates a bias on the basis of the source potential at the point B acquired from the feedback path 912 and the reference potential output from the power supply 911.

For example, the bias generator circuit 132 generates the bias so that a difference between the source potential and the reference potential decreases. In this manner, the bias generator circuit 132 can generate a bias Vbs for compensating the fluctuation of the threshold voltage Vth of each of the transistors 124 a and 124 b due to process variation.

FIG. 10 is a circuit diagram illustrating a modification of the semiconductor integrated circuit illustrated in FIG. 9. In FIG. 10, like reference characters are attached to configurations similar to those illustrated in FIG. 9 to omit the description. As illustrated in FIG. 10, the semiconductor integrated circuit 100 may include a comparator circuit 1011 in addition to the components illustrated in FIG. 9. The feedback path 912 is connected to the comparator circuit 1011. The power supply 911 outputs the reference potential to the comparator circuit 1011.

The comparator circuit 1011 compares the source potential acquired from the feedback path 912 with the reference potential output from the power supply 911. Upon determining that the source potential is higher than the reference potential, the comparator circuit 1011 outputs the source potential and the reference potential to the bias generator circuit 132. Upon determining that the source potential is not higher than the reference potential, the comparator circuit 1011 outputs neither the source potential nor the reference potential to the bias generator circuit 132.

Accordingly, the bias generator circuit 132 generates the bias if the comparator circuit 1011 determines that the source potential is higher than the reference potential. However, the bias generator circuit 132 does not generate the bias if the comparator circuit 1011 determines that the source potential is not higher than the reference potential. In this manner, the bias can be applied to substrates of the transistors 124 a and 124 b when the threshold voltages V_(th) of the transistors 124 a and 124 b are higher than the reference voltage, respectively.

The semiconductor integrated circuit 100 according to the third embodiment detects the threshold voltages V_(th) of the transistors 124 a and 124 b by detecting the source potentials of the transistors 124 a and 124 b that fluctuate with the threshold voltages V_(th), respectively. In this manner, the threshold voltage V_(th) of each of the transistors 124 a and 124 b can be accurately detected. The semiconductor integrated circuit 100 according to the third embodiment can accurately compensate the fluctuation of the threshold voltage V_(th) due to process variation by generating the bias on the basis of the detected source potential and the reference potential and, eventually, can increase a margin of circuit design.

(Upper Limit of Substrate Bias)

FIG. 11 is a graph illustrating a characteristic of current flowing through a substrate of a transistor against a bias. Referring to FIG. 11, the horizontal axis represents a bias V_(bs) [V] applied to the substrates of the transistors 124 a and 124 b, whereas the vertical axis represents current [A] flowing through the substrates of the transistors 124 a and 124 b, respectively. A curve 1100 represents a characteristic of current flowing through the substrate of each of the transistors 124 a and 124 b against the bias Vbs.

For example, it is assumed that threshold voltages of PN junctions of the transistors 124 a and 124 b are equal to 0.6 V. In this case, as illustrated by the curve 1100, current flows through the substrates of the transistors 124 a and 124 b if a bias equal to or higher than 0.6 V is applied to the substrates of the transistors 124 a and 124 b, respectively. When the current flows through the substrates of the transistors 124 a and 124 b, signals output from the output terminals 126 a and 126 b deteriorate because of an influence of the current.

Accordingly, the bias generator circuit 132 according to the foregoing embodiments may generate the bias not exceeding the threshold voltages (0.6 V in this example) of the PN junctions of the transistors. The semiconductor integrated circuit 100 can reduce, if not prevent, current from flowing through the substrates of the transistors 124 a and 124 b by generating the bias not exceeding the threshold voltages of the PN junctions of the transistors 124 a and 124 b and, eventually, can improve a quality of signals output from the output terminals 126 a and 126 b, respectively.

(Examples of Other Semiconductor Integrated Circuits)

The transistors 124 a and 124 b, the detector circuit 131, and the bias generator circuit 132 can be applied not only to the semiconductor integrated circuit 100 but also to various semiconductor integrated circuits involving DC potential design. For example, the transistors 124 a and 124 b, the detector circuit 131, and the bias generator circuit 132 may be applied to the selector 620 illustrated in FIG. 6 (see FIG. 12).

FIG. 12 is a circuit diagram illustrating a first example of other semiconductor integrated circuits. In FIG. 12, like reference characters are attached to configurations similar to those illustrated in FIG. 1 to omit the description. A semiconductor integrated circuit 1200 illustrated in FIG. 12 includes a power supply 1201, paths 1202 a and 1202 b, input terminals 1203 a, 1203 b, 1204 a, 1204 b, 1205 a, and 1205 b, output terminals 1240 a and 1240 b, an amplifier circuit 614, a selector 620, a bias generator circuit 132, a monitor transistor 711, and a power supply 712.

The power supply 1201 supplies voltage to the amplifier circuit 614 and the selector 620. A clock signal amplified by the amplifier circuit 614 is output to the selector 620 through the paths 1202 a and 1202 b. The input terminals 1203 a and 1203 b correspond to the input terminal 613 illustrated in FIG. 6. A non-inverted clock signal (CLK) is input to the input terminal 1203 a, whereas an inverted clock signal (CLKx) is input to the input terminal 1203 b.

The amplifier circuit 614 includes a resistor 1211, resistors 1212 a and 1212 b, transistors 1213 a and 1213 b, and a current source 1214. One end of the resistor 1211 is connected to the power supply 1201, whereas the other end thereof is connected to the resistors 1212 a and 1212 b. One end of the resistor 1212 a is connected to the resistor 1211, whereas the other end thereof is connected to a drain of the transistor 1213 a. One end of the resistor 1212 b is connected to the resistor 1211, whereas the other end thereof is connected to a drain of the transistor 1213 b.

The transistors 1213 a and 1213 b are included in a differential pair. The drain of the transistor 1213 a is connected to the resistor 1212 a. A source of the transistor 1213 a is connected to the current source 1214, whereas a gate of the transistor 1213 a is connected to the input terminal 1203 a. The drain of the transistor 1213 b is connected to the resistor 1212 b. A source of the transistor 1213 b is connected to the current source 1214, whereas a gate of the transistor 1213 b is connected to the input terminal 1203 b.

One end of the current source 1214 is connected to the sources of the transistors 1213 a and 1213 b, whereas the other end thereof is connected to ground. The path 1202 a to the selector 620 is connected between the resistor 1212 a and the transistor 1213 a. The path 1202 b to the selector 620 is connected between the resistor 1212 b and the transistor 1213 b.

The clock signals input from the input terminals 1203 a and 1203 b are applied to the gates of the transistors 1213 a and 1213 b, respectively. In accordance with the input clock signals, current from the resistors 1212 a and 1212 b to the current source 1214 is generated. Accordingly, the amplified clock signals are output to the selector 620 through the paths 1202 a and 1202 b.

The input terminals 1204 a and 1204 b correspond to the input terminal 611 illustrated in FIG. 6. A non-inverted data signal (DATA1) is input to the input terminal 1204 a, whereas an inverted data signal (DATA1 x) is input to the input terminal 1204 b. The input terminals 1205 a and 1205 b correspond to the input terminal 612 illustrated in FIG. 6. A non-inverted data signal (DATA2) is input to the input terminal 1205 a, whereas an inverted data signal (DATA2 x) is input to the input terminal 1205 b.

The selector 620 includes resistors 1221 a and 1221 b, transistors 1222 a, 1222 b, 1223 a, and 1223 b, a current source 1224, resistors 1231 a and 1231 b, transistors 1232 a, 1232 b, 1233 a, and 1233 b, and output terminals 1240 a and 1240 b.

One end of the resistor 1221 a is connected to the power supply 1201, whereas the other end thereof is connected to a drain of the transistor 1222 a. One end of the resistor 1221 b is connected to the power supply 1201, whereas the other end thereof is connected to a drain of the transistor 1222 b.

The transistors 1222 a and 1222 b are included in a differential pair. The drain of the transistor 1222 a is connected to the resistor 1221 a. A source of the transistor 1222 a is connected to a drain of the transistor 1223 a, whereas a gate of the transistor 1222 a is connected to the input terminal 1204 a. The drain of the transistor 1222 b is connected to the resistor 1221 b. A source of the transistor 1222 b is connected to a drain of the transistor 1223 b, whereas a gate of the transistor 1222 b is connected to the input terminal 1204 b.

The transistors 1223 a and 1223 b are included in a differential pair. The drain of the transistor 1223 a is connected to the source of the transistor 1222 a. A source of the transistor 1223 a is connected to the current source 1224, whereas a gate of the transistor 1223 a is connected to the path 1202 a. The drain of the transistor 1223 b is connected to the source of the transistor 1222 b. A source of the transistor 1223 b is connected to the current source 1224, whereas a gate of the transistor 1223 b is connected to the path 1202 a.

One end of the resistor 1231 a is connected to the power supply 1201, whereas the other end thereof is connected to a drain of the transistor 1232 a. One end of the resistor 1231 b is connected to the power supply 1201, whereas the other end thereof is connected to a drain of the transistor 1232 b.

The transistors 1232 a and 1232 b are included in a differential pair. The drain of the transistor 1232 a is connected to the resistor 1231 a. A source of the transistor 1232 a is connected to a drain of the transistor 1233 a, whereas a gate of the transistor 1232 a is connected to the input terminal 1205 a. The drain of the transistor 1232 b is connected to the resistor 1231 b. A source of the transistor 1232 b is connected to a drain of the transistor 1233 b, whereas a gate of the transistor 1232 b is connected to the input terminal 1205 b.

The transistors 1233 a and 1233 b are included in a differential pair. The drain of the transistor 1233 a is connected to the source of the transistor 1232 a. A source of the transistor 1233 a is connected to the current source 1224, whereas a gate of the transistor 1233 a is connected to the path 1202 b. The drain of the transistor 1233 b is connected to the source of the transistor 1232 b. A source of the transistor 1233 b is connected to the current source 1224, whereas a gate of the transistor 1233 b is connected to the path 1202 b.

Each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b may be, for example, a MOS transistor. Each of the transistors 1213 a, 1213 b, 1222 a, 1222 b, 1232 a, and 1232 b may also be, for example, a MOS transistor. A bias (substrate bias) output from the bias generator circuit 132 is applied to a substrate of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b.

The output terminal 1240 a is connected between the resistor 1221 a and the drain of the transistor 1222 a and between the resistor 1231 a and the drain of the transistor 1232 a. The output terminal 1240 b is connected between the resistor 1221 b and the drain of the transistor 1222 b and between the resistor 1231 b and the drain of the transistor 1232 b.

The non-inverted clock signal (CLK) output from the amplifier circuit 614 through the path 1202 a is applied to the gates of the transistors 1223 a and 1223 b. The data signals (DATA1 and DATA1 x) input from the input terminals 1204 a and 1204 b are applied to the gates of the transistors 1222 a and 1222 b, respectively. In accordance with the non-inverted clock signal (CLK) and the data signals (DATA1 and DATA1 x), current from the resistors 1221 a and 1221 b to the current source 1224 is generated.

The inverted clock signal (CLKx) output from the amplifier circuit 614 through the path 1202 b is applied to the gates of the transistors 1233 a and 1233 b. The data signals (DATA2 and DATA2 x) input from the input terminals 1205 a and 1205 b are applied to the gates of the transistors 1232 a and 1232 b, respectively. In accordance with the inverted clock signal (CLKx) and the data signals (DATA2 and DATA2 x), current from the resistors 1231 a and 1231 b to the current source 1224 is generated.

Accordingly, the data signals (DATA1 and DATA1 x) are output from the output terminals 1240 a and 1240 b in synchronization with the non-inverted clock signal (CLK), respectively. The data signals (DATA2 and DATA2 x) are output from the output terminals 1240 a and 1240 b in synchronization with the inverted clock signal (CLKx), respectively. In this manner, the selector 620 alternately selects and outputs one of the pair of data signals input from the input terminals 1204 a and 1204 b and the pair of data signals input from the input terminals 1205 a and 1205 b in synchronization with the clock signals (CLK and CLKx) input from the input terminals 1203 a and 1203 b, respectively.

The selector 620 includes two stages, namely, one stage including the transistors 1222 a and 1222 b and the other stage including the transistors 1223 a and 1223 b. Accordingly, in the semiconductor integrated circuit 1200, a fluctuation of threshold voltages V_(th) of the transistors 1223 a, 1223 b, 1233 a, and 1233 b due to process variation decreases a margin in DC potential design at a point b (between the current source 1224 and the transistors 1223 a, 1223 b, 1233 a, and 1233 b).

For example, if the threshold voltage V_(th) of each of the transistors 1223 a and 1223 b increases because of process variation, a potential at the point b approaches 0 V, and voltage for driving the current source 1224 becomes insufficient. Accordingly, the DC potential design of the selector 620 fails.

However, in the semiconductor integrated circuit 1200, the monitor transistor 711 and the bias generator circuit 132 compensate the fluctuation of the threshold voltage V_(th) of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b due to process variation. More specifically, the monitor transistor 711 detects the threshold voltage of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b.

The monitor transistor 711 monitors the threshold voltage V_(th) of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b. The monitor transistor 711 may be, for example, a MOS transistor having substantially the same current density as the transistors 1223 a, 1223 b, 1233 a, and 1233 b. The monitor transistor 711 is formed on a chip including the transistors 1223 a, 1223 b, 1233 a, and 1233 b.

The bias output from the bias generator circuit 132 is applied to a substrate of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b. In this manner, the fluctuation of the threshold voltage V_(th) due to process variation of the transistors 1223 a, 1223 b, 1233 a, and 1233 b can be compensated. Accordingly, the semiconductor integrated circuit 1200 can increase the margin in the DC potential design at the point b, for example. The transistors 1223 a and 1223 b correspond to, for example, the transistors 124 a and 124 b illustrated in FIG. 1, respectively. Additionally, the transistors 1233 a and 1233 b correspond to the transistors 124 a and 124 b, respectively.

FIG. 13 is a circuit diagram illustrating a second example of other semiconductor integrated circuits. In FIG. 13, like reference characters are attached to configurations similar to those illustrated in FIG. 12 to omit the description. A semiconductor integrated circuit 1200 illustrated in FIG. 13 corresponds to the semiconductor integrated circuit 1200 illustrated in FIG. 12 further including the power supply 911 and the feedback path 912 illustrated in FIG. 9.

The power supply 911 outputs, to the bias generator circuit 132, a source potential (reference potential) of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b resulting from a process without process variation.

The potential at the point b of the semiconductor integrated circuit 1200 is fed back to the bias generator circuit 132 through the feedback path 912. The bias generator circuit 132 generates a bias so that a difference between the source potential and the reference potential decreases. In this manner, the bias generator circuit 132 can generate a bias V_(bs) for compensating the fluctuation of the threshold voltage V_(th) of each of the transistors 1223 a, 1223 b, 1233 a, and 1233 b due to process variation.

As described above, the semiconductor integrated circuits can increase a margin of circuit design.

Advantageously, the disclosed semiconductor integrated circuits can provide a benefit of increasing a margin of circuit design.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A semiconductor integrated circuit comprising: a cascode circuit including a transistor, a bias being applied to a substrate of the transistor; a detector circuit to generate a signal related to a threshold voltage of the transistor; and a bias generator circuit to generate the bias based on the signal generated by the detector circuit.
 2. The semiconductor integrated circuit according to claim 1, wherein the transistor is a metal oxide semiconductor (MOS) transistor.
 3. The semiconductor integrated circuit according to claim 1, wherein the bias generator circuit generates the bias based on both the signal generated by the detector circuit and a reference voltage for the transistor.
 4. The semiconductor integrated circuit according to claim 2, wherein the detector circuit includes a monitor transistor that is disposed on the same chip as the transistor and has substantially the same current density as the transistor, and the detector circuit is further operable to detect a threshold voltage of the monitor transistor.
 5. The semiconductor integrated circuit according to claim 4, further comprising: a comparator circuit to compare the threshold voltage of the monitor transistor with the reference voltage, wherein the bias generator circuit is further operable to generate the bias when the comparator circuit determines that the threshold voltage is higher than the reference voltage, whereas the bias generator circuit is further operable to not generate the bias when the comparator circuit determines that the threshold voltage is equal to or lower than the reference voltage.
 6. The semiconductor integrated circuit according to claim 2, wherein the bias generator is connected to a source of the transistor, and the detector circuit is further operable to detect a voltage-potential of the source of the transistor.
 7. The semiconductor integrated circuit according to claim 6, wherein the bias generator circuit is further operable to generate the bias on the basis of the voltage-potential of the source detected by the detector circuit and a reference-potential for the transistor.
 8. The semiconductor integrated circuit according to claim 7, wherein the bias generator circuit is further operable to generate the bias so as to decrease a difference between the source potential and the reference-potential.
 9. The semiconductor integrated circuit according to claim 7, further comprising: a comparator circuit to compare the voltage-potential of the source with the reference-potential, wherein the bias generator circuit is further operable to generate the bias when the comparator circuit determines that the voltage-potential of the source is higher than the reference-potential, whereas the bias generator circuit is further operable to not generate the bias when the comparator circuit determines that the voltage-potential of the source is equal to or lower than the reference-potential.
 10. The semiconductor integrated circuit according to claim 1, wherein the bias generator circuit is further operable to generate the bias to be equal to or lower than a threshold voltage of a PN junction of the transistor.
 11. The semiconductor integrated circuit according to claim 1, wherein the transistor is included in a downstream-side circuit in a multi-stage circuit.
 12. A semiconductor integrated circuit comprising: a cascode circuit including an first transistor and a second transistor, a bias being applied to a substrate of the second transistor; a detector circuit to indirectly detect a threshold voltage of the second transistor and to provide a signal indicative thereof; and a bias generator circuit to generate the bias based on the signal generated by the detector circuit.
 13. The semiconductor integrated circuit according to claim 12, wherein the detector circuit includes a monitor transistor that has substantially the same current density as the second transistor and is formed on the same chip as the second transistor in a location sufficiently proximal to the second transistor such that the monitor transistor can be regarded as having been formed according to substantially the same process variations which formed the second transistor, and the detector circuit is further operable to directly detect a threshold voltage of the monitor transistor, the indirectly detected threshold voltage of the second transistor being represented by the directly detected threshold voltage of the monitor transistor. 